The present invention relates to the installation of tensioned reinforcements, such as strands, within a sheath, for the production of a stay belonging to the suspension system of a civil engineering structure.
In a stayed suspension, one or more pylons support a structure, such as, for example, a bridge deck, by means of a set of stays following oblique paths between a pylon and the structure. A stay is a cable composed of a set of reinforcements tensioned between two end anchorages and generally surrounded by a sheath. These reinforcements are often metal strands. Where a stayed bridge is concerned, each reinforcement is anchored on a pylon and on the deck of the bridge which it participates in supporting.
European Patent 0 421 862 describes a method for the tensioning of the strands of a stay, which advantageously makes it possible to balance the tensions between the various strands, whilst at the same time using a single-strand jack which is much lighter and manipulatable (above all, on a pylon) than a collective jack. According to this method, a first strand is tensioned in order to form a reference strand. Each following strand is tensioned with the aid of the single-strand jack, until its tension has the same value as that of the reference strand. During this operation, the tension of the strands already anchored decreases somewhat at the same time as that of the new strand increases. This operating mode progressively ensures that the various strands of the stay will be tensioned to the same value.
For large-size structures, the stays used are typically of great length, which may amount to several hundred metres, and a high number of tensioned elementary reinforcements (strands or the like) must be provided in order to support the load.
Moreover, on stayed structures with a very wide span (greater than 500 metres), the drag force on the sheet of stays predominates over the action of the wind on the deck and can lead to overdimensioning of the pylons. As the drag is proportional to the diameter of the sheath, it is therefore desirable to provide stays with a reduced-diameter sheath, that is to say more compact stays.
A difficult compromise must therefore be found between the number of strands per stay, which it is desirable to maximize in order to increase the supporting capacity of the stay, and its diameter, which it is desirable to minimize for aerodynamic reasons.
It is generally necessary, then, to provide space in the sheath in order to cause reinforcements to circulate during the installation of the stay. To be precise, the stays of large bridges are very heavy, so that it is not conceivable to hoist them after they have been prefabricated on the deck or on a prefabrication zone. In general, the sheath is put in place along the oblique path of the stay, and then the strands are installed one by one or small group by small group, by hoisting them with the aid of a shuttle sliding in the sheath and driven by a winch located on the pylon. During the hoisting of the last strand (or of the last group), sufficient space must remain in the sheath to allow the shuttle to pass. It is clearly desirable to minimize this residual space with a view to reaching the above compromise.
In EP-A-0 654 562, this problem was bypassed, in that the sheath consists of a plurality of shells assembled around a bundle of strands after the tensioning of these, thus making it possible to leave only a minimal space. However, for the general design of the stay, it is definitely preferable to provide a sheath in one piece, rather than in a plurality of parts. This, in particular, affords better protection with reinforcements against environmental attacks.
Moreover, it is necessary to ensure that the strands installed in the sheath are in parallel. This is carried out, in particular, by positioning the strands in a paired manner in the top and bottom anchorages by means of a numbering of the anchoring holes. But, there is a risk that a shuttle would tend to rotate on itself during hoisting, thus causing the hoisted strands to be entangled with one another and with the small cables used for ensuring the displacement of the shuttle between the anchoring zones. This applies particularly when the cross section of the shuttle used is smaller than that of the hoisted group of strands. This situation therefore necessitates the use of relatively bulky shuttles, thus making necessary, furthermore, to reserve a corresponding space in the sheath to allow the passage of such shuttles.
An object of the present invention is to provide a satisfactory solution to the problem set out above.